Indium Doped Zinc Oxide Thin Films - jpr solutions

International Journal of Chemical and Analytical Science ISSN: 0976-1206 Research Article www.ijcas.info

Indium Doped Zinc Oxide Thin Films Pankaj Yadav1 , Hiren C Mandalia2*, Chintan Pathak3, Kavita Pandey1 1Department of Physics & Electronics, Gujarat University, Ahmedabad- 380 009, Gujarat, India 2R & D Centre, Gujarat Narmada Valley Fertilizer Company Ltd. (GNFC), Narmadanagar - 392 015, Bharuch, Gujarat, INDIA 3School of Technology, Pandit Deendayal Petroleum University, Gandhinagar-382 007, Gujarat, INDIA Undoped and Indium doped ZnO films have been deposited by sol–gel spin coating method. ZnO films were deposited on silica glass substrates. Both doped and undoped samples were analyzed and results were discussed through XRD data. The XRD of the film confirms the effect of Indium incorporation on structural and optical properties of ZnO films. X-ray diffraction patterns of the films showed the Hexagonal Wurtzite type polycrystalline structure and that Indium incorporation leads to substantial changes in the structural characteristics of ZnO films. Electrical resistivity decreased from 10.03 to 0.503 ohm cm due to doping of Indium. Lattice constant a and c change almost linearly with change in the concentration of Indium in ZnO thin films. We observe that as the grain size decreases the dislocation density increases. With the change in the Indium concentration from 0 to 9 mg the band gap reduces from 3.24 to 3.18 eV this confirm in increase the conductivity of ZnO thin films with doping of Indium. The presented results show that the obtained thin films can be used as a photovoltaic material. Keywords: Indium-doped ZnO thin film, Van der Pauw method, X-ray diffraction.

INTRODUCTION The application of thin film in modern technology is widespread. The methods employed for thin film deposition can be divided into two groups based on the nature of deposition viz, physical or chemical. The physical method includes physical vapor deposition (PVD), laser ablation, molecular beam epitaxy, and sputtering. The chemical method comprises gas-phase deposition method and solution techniques. The gas phase methods are chemical vapor deposition (CVD) and atomic layer epitaxy (ALE), while spray pyrolysis, sol gel, spin coating and dip-coating methods employ precursor solution. ZnO is a versatile material, therefore it is not surprising that it has been under intensive investigation for a long time since 1950s Thin- film zinc oxide continues to attract because of its low toxicity and a wide range of technical application such as transparent conducting electrode in solar cell1, surface acoustic wave device2 , chemical sensor 3, varistor 4, and electro-and photo luminescent devices. The energy band window has promoted many studies on the preparation and characterization of less expensive and stable transparent conducting oxides (TCOs). Many reports were devoted to fabrication of ZnO film. Highquality ZnO film was produce by magnetron sputtering5 , laser ablation6, metal- organic chemical-vapor deposition7 , laser molecular-beam epitaxy8, successive ion layer adsorption and reaction9 and electrochemical synthesis10.

Temperature and continuous spray unit which can spray 1 ml/min. Doped and undoped ZnO thin film with Indium film thickness measured with the help of surface profilometer (DEKTAK). Film is characterized with XRD and stress is calculated at different axis and resistivity of film is measured with the help of Hall Effect. The presented results demonstrate that the modified ZnO thin films in the course of Indium can be used as a photovoltaic objects.

ZnO thin films doped with a wide variety of elements are commonly reported in literature.11 The ZnO films synthesis includes three principal steps: (i) solution preparation, (ii) coating and (iii) heat treatment. For the first step, the particle formation is discussed including nucleation and growth, particle size, morphology and colloids stability. The aim is to decrease the resistivity, to increase the transparency and to enhance chemical stability. Until now, there has been much work reported on the electrical and optical properties12 . However, we have noted in the literature the absence of systematic work related to uniaxial stress influence on ZnO lattice films through Indium doping.

Fig.1. Microcontroller interfacing with spin unit and syringe

Design using microcontroller: In a usual manual spin coating one has to regulate and observe the spin start and stop, deposit fluid in substrate, accelerate wafer to final radial velocity, evaporation of solvent from film and substrate motion. During spray pyrolysis one has to regulate and observe the spray on, off duration, ventilation, carrier gas flow and substrate motion till the thin film coating is complete. A microcontroller device can easily and efficiently handle this task. This will ensure better reproducibility of thin film. The block diagram of microcontroller and its interface with the rotating spin disk and syringe is given in Fig.1. The interface diagram of microcontroller and motor shown in Fig. 2. The Printed circuit board (PCB) layout shown in Fig. 3. The photograph of the microcontroller is shown in Fig. 4. The circular rotation of the substrate helps in spreading the splashing droplet on it due to centrifugal force13.

The objective and vision of this research article is designed and developed a spin coating unit with control rpm of disc,

Corresponding Author: Dr. Hiren C Mandalia, R & D Centre, Gujarat Narmada Valley Fertilizer Company Ltd. (GNFC), Narmadanagar - 392 015, Bharuch, Gujarat, INDIA. Received 17-12-2011; Accepted 25-01-2012 March, 2012

International Journal of Chemical and Analytical Science, 2012, 3(3), 1352-1356

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Pankaj Yadav, et al.: Indium Doped Zinc Oxide Thin Films

Fig. 2.The interface diagram of microcontroller and motor

and 9 mg) where used for doping different sets of ZnO films. These doped sample were named ZnO:In(1), ZnO: In(2) and ZnO:In(3), respectively. The crystal structure of the films was analyzed with BRUKER Series X-Ray Automatic Diffractometer with CuKα (λ=1.54059 Å) radiation, at room temperature. Film deposition: The film was deposited on the glass substrate which was cleaned as describe above. The solution is loaded into a 5 ml syringe that was equipped with 23-Gsized metal nozzle. The distance between solution loaded tip and the substrate were maintained at 5 cm. The solution was injected using syringe pump which is controlled by microcontroller at the rate of 0.9 ml/min. For the spin coated films, the solution was spin coated on a glass substrate at 3000 rpm for 45 min at 95˚C. After the deposition of film the film was annealed at 350˚C for 1 hour. Preparation of solution and deposition of film were carried out under approximately identical conditions. The films obtained were crack-free and homogeneous. The films of higher thickness were obtained by repeating the process of coating and drying. During deposition, the film thickness can be roughly estimated by monitoring the cyclic apparition of the interface colors of the light refracted from the film. The growth in all the cases stopped when the green color interference appeared14 .

Fig. 3.The PCB layout

RESULTS AND DISCUSSION

Fig. 4. Image of microcontroller interface with stepper motor

Experimental Details: In case of wide-band-gap semiconductor, addition of impurities often induce dramatic changes in their electrical and optical properties. For Indium doping commonly used doping solution is Indium chloride. But when Indium chloride was used for doping, unintentional doping of chlorine was also taken place. Our main objective was to avoid presence of chlorine being incorporate into the sample. The glass substrates were cleaned by deionized (DI) water, acetone and isopropyl alcohol bath for 20 min each and dried at 90oC for 10 min prior to use. The substrates were, then, UV/ozone treated for 20 min. The precursor materials were zinc acetate dehydrate, Zn (CH3 COO)2 .2H2 O (99.99% purchased from Fluka) and Indium acetate, In(CH3 COO) 3 (99.99% purchased from Fluka) and 2Methoxyethanol (purchased from Fluka) used to provide the viscosity to the solution. Three different sets of Indium (3, 6,

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Structural Properties: The structure of the film was studies with x-ray diffraction (XRD) pattern obtained from a x-ray beam (Philips PW-1710) of wave length λ=1.5404Ǻ. A continuous mode scan was used to collect 2 theta data with 4o per scan rate. Fig. 5 shows the x-ray diffraction pattern of the undoped ZnO thin film. Fig. 6, 7, 8 shows the x-ray diffraction pattern of the Indium doped ZnO: In(x) thin film. It is found that all films show preferential orientation along (002) plane. It was shown that the intensity of the peak corresponding to (002) plane has a strong dependence on the oxygen content in the sample. 15 New phases where observed when the mass of the Indium was increased up to 9mg, this indicate that incorporation of the Indium could not change the wurtzite structure of ZnO. From Fig. 8, It is clear that the peak position to the plane (002) and (001) shifted to lower value of 2theta, when Indium was incorporated. The average grain size D was calculated using the Scherrer’s formula16.

In which λ is the x-ray wavelength, θ is the diffraction angle, ∆(2θ) is the line width at half maximum in excess of the instrumental broadening, and K= 0.94 is a constant depending slightly on the crystal shape. The dislocation density was calculated by the relation: δ = 1/D 2 Where D is the grain size from the dislocation density we can understand the degree of crystallinity. The Microstrain was calculated by the formula ɛ = ∆ (2θ) cosθ/4


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For hexagonal lattice the strain along a-, b- and c- axis can be calculated by ea = eb = (a-ao)/ao and ec = (c-co)/co ,

Fig. 8, XRD spectra of ZnO: In (3)

Where ao and co are the lattice parameter of the stress-free bulk ZnO where as a and c are the lattice parameter of the ZnO film. Fig.9 shows primitive cell structure of wurtzite ZnO. The calculated lattice parameters to the plane (002) as well as calculated lattice constant a and c under different uniaxial stress are presented in Table 1 and Table 2 respectively. The grain size of crystallites was calculated using a wellknown Scherrer’s formula.16 The grain size values are given in Table 1 found to be 258.1 Ǻ, 186.6 Ǻ, 180.0 Ǻ and 179.2 Ǻ for ZnO, ZnO:In(1), ZnO:In(2) and ZnO:In(3), respectively. It can be seen that as long as the indium content increases the grain size of the films decreases. The stress along the axis is due to the ionic radius of Indium (0.080nm) is larger than the Zn ion (0.074nm). The lattice constant c increase with the increase of uniaxial tensile stress.

Fig.9 Primitive cell structure of wurtzite ZnO

Fig. 5. XRD spectra of ZnO

Table 1 The lattice parameters corresponding to the plane (002) Parameter/ sample

d (Ǻ)

Lattice parameters

Fig. 6 XRD spectra of ZnO: In (1) Ideal ZnO ZnO

Grain Size (Ǻ)

a

c

c/a

2.6033

3.249

5.206

1.602

2.6050

3.24

5.210

1.608

258.2

2.6064

3.24

5.212

1.608

186.6

2.6084

3.251

5.216

1.604

180.0

2.6084

3.253

5.216

1.600

179.2

ZnO:In(1)

ZnO:In(2) ZnO:In(3)

Table 2 Calculated stress along the different axis Parameter/ Sample

Fig. 7 XRD spectra of ZnO: In (2)

Dislocation Density Line2/m2

Stress along ‘a’ axis

Stress along ‘c’ axis

Micro strain (ɛ)

ZnO 1.499*10 -15

-------

0.768*10-3

1.39*10 -3

2.870*10 -15

-------

1.15*10 -3

1.91*10 -3

3.086*10 -15

0.307*10-3

1.92*10 -3

2.00*10 -3

3.114*10 -15

0.338*10-3

1.92*10 -3

2.00*10 -3

ZnO:In(1)

ZnO:In(2) ZnO:In(3)

Fig. 10, shows the variation of the uniaxial stress along c and a axis of ZnO doped and undoped thin films from graph it is concluded that with the stress along c axis is increase with the increase in the weight of Indium is comparable more than the a axis.

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Fig.10 Change in stress along axis due to Indium weight change

Pauw method17. The electrical resistivity of ZnO films as a function of indium content are plotted in Fig. 13. Fig.13 Electrical resistivity of ZnO thin film at different Indium concentration

Fig. 11 shows the variation between grain size and dislocation density with the change in the weight of the Indium from figure it is concluded that with the increase in grain size the grain boundary barrier reduces and degree of crystallinity in deposited films increases. Fig. 11 Grain size/dislocation density vs. wt of Indium

Fig.12 shows the variation of the Microstrain in the ZnO film with the change of dopant amount. Fig.12 Microstrain vs. wt of Indium

It is shown that indium content is a parameter affects the electrical properties of these films. It is seen that as indium is incorporated to the ZnO films, the electrical resistivity first decreases and then increases. The lowest resistivity value was obtained for the films doped with 1% of In. An initial decrease in the resistivity is due to an increase in the freeelectron concentration with indium incorporation in the ZnO film. In the other words, it can be attributed to the optimal incorporation of indium atoms into the lattice, increasing the donor concentration and contributing to a decrease of the resistivity. By adding more indium in the ZnO film, at high indium concentration will increase the scattering process and therefore the majority carrier mobility is decreased. It is also observed that electrical resistivity of these films decreased when exposed to UV-illumination. This indicates that the UVillumination increases the production of electron-hole pairs. So, the obtained ZnO thin films can be used as a photovoltaic material. Electrical resistivity measurement were done on both doped and undoped sample and found it to be decrease with Indium doping Fig. 13 depicts the variation of the resistivity with Indium concentration. The decrease in resistivity might be naturally due to the donor action of Indium. On doping, Zinc might be replaced by Indium atom and since it belongs to III group in periodic table it could supply additional atom contributing the conductivity. Optical property: The dependence of the band gap of doped and undoped Zno films on the Indium weight is shown in Fig.14 it shows that the band gap reduce from 3.29 to 3.19 (eV) as the Indium concentration increase from 0 to 9 mg This decrease in the band gap can be due to the influence of various factors such as grain size, structural parameters, carrier concentration, presence of impurities, deviation from stiochiometry of the film and lattice strain18,19 however we have observed that lattice parameter, grain size and liner stress along the axis have a direct relation with the concentration of the Indium in ZnO thin films.

Electrical resistivity measurements: Hot probe method was used in order to verify conductivity type. It was seen that all the films exhibit n-type conductivity, which is attributed to a deviation from stoichiometry, due to the oxygen vacancies and interstitial zinc and/or indium atoms. Both vacancies and interstitial atoms act as donors. The electrical resistivity values of ZnO and Indium doped ZnO films in dark and under UV-illumination were calculated by using Van der

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Fig.14 Band gap of ZnO thin films at different Indium concentration

CONCLUSION ZnO films were prepared using the spin-coating technique, were doped with different concentration of Indium. Different quantities of Indium (3, 6, and 9 mg) were deposited on ZnO films and films were annealed at 100˚C for 1 hour. Both doped and undoped samples were characterized using different techniques and the results are systematically presented. From the X-ray diffraction analysis all the film shows hexagonal structure along with c axis oriented (002) plane. Electrical resistivity decreases from 10.03 to 0.503 ohm cm due to doping. Lattice constant a and c change almost linearly with change in the concentration of Indium in ZnO thin films. We observe that as the grain size decreases the dislocation density increases. As lattice constant change linearly we may conclude that film become more porous in nature. This modified film has characteristics required for application of suitable for the solar cell applications.

ACHNOWLEDGEMENT One of the authors, Mr. Pankaj Yadav has carried out this work during his Master of Science course and gratefully acknowledges Department of Physics, Electronics and Space Science of Gujarat University, Ahmedabad, India for providing financial assistance, instrumental facilities and INFLIBNET, Ahmedabad, India for providing online journals.

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Source of support: Electronics and Space Science of Gujarat University, Conflict of interest: None Declared

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